Beta-alumina was first reported by Rankin and Merwin in 1916. However, extensive research on this material did not take place until fifty years later, when its use as a solid electrolyte for the sodium-sulfur battery was investigated. Sodium Beta-alumina exhibits the unique property of having a high sodium ion conductivity at 300.degree. C., which is the temperature at which the sodium-sulfur battery is operated.
Sodium Beta-alumina is a highly ion-conductive ceramic which can be represented by the chemical formula: Na.sub.2 O.xAl.sub.2 O.sub.3, where x can vary between 5 and 11. Sodium Beta-alumina powder is generally produced by reaction fusion of Al.sub.2 O.sub.3 powder with Na.sub.2 O in the form of a sodium salt, at 1250.degree. C. to 1500.degree. C., to form Na.sub.2 O.xAl.sub.2 O.sub.3, as described by Iijima, in U.S. Pat. No. 4,082,826. The sodium Beta-alumina powder is then formed into the desired electrolyte shape, and sintered at up to 1750.degree. C. DeJonghe et al., in U.S. Pat. No. 3,959,022, reduced sintering temperatures by first forming a eutectic mixture of sodium-aluminum oxide, in which the atom ratio of sodium to aluminum is 0.54. This eutectic mixture is added to sodium Beta-alumina powder and the additive mixture is heated at about 1600.degree. C., to achieve reactive liquid phase sintering.
The grain size, purity and porosity of sodium Beta-alumina are extremely important factors influencing its ion conductivity and performance as a solid electrolyte. The high temperature fusion method of formation, where Al and Na are reacted in the solid state at from 1250.degree. C. to about 1550.degree. C., generally entails undesirable grain growth and purity problems, and requires sintering at up to 1800.degree. C., further increasing grain growth, which increases porosity.
In addition, bulk sodium Beta-alumina does not have very good thermal or electrical shock resistance, and cannot be formed easily into thin sections. When formed into a solid electrolyte for battery applications, fracturing due to mechanical or thermal shock is possible, allowing contact of molten sulfur and molten sodium with catastrophic results. Such thick solid electrolytes, made solely of thick sodium Beta-alumina, will also exhibit high electrical resistance. What is needed is a method of making thin, continuous, and essentially impermeable films of sodium Beta-alumina, which could be coated onto suitable high strength supporting substrates. Such a thin film system could still provide a Na.sup.+ conductive barrier between molten sodium and molten sulfur in a battery electrolyte, and could also have excellent resistance against mechanical failure.
To this end, Toy et al, in U.S. Pat. No. 3,901,733, formed 50 to 1000 micron thick, Beta-alumina barrier layer films on polyaluminate support sheets, where leakage of molten sodium was controlled by film thickness. The film was applied by either high temperature plasma arc spraying, radio frequency ion sputtering, chemical vapor deposition, pressing a layer of dense Beta-alumina powder onto the support and then sintering at 1500.degree. C. to 1880.degree. C., or spraying a slurry of crystalline Beta-alumina powder having a neutral surface charge onto the support and then sintering at 1500.degree. C. to 1800.degree. C. Such a method still uses crystalline particulate application techniques in some form, and either requires expensive and sophisticated high temperature application means, or final sintering at between 1500.degree. C. and 1800.degree. C., increasing energy consumption and requiring expensive furnaces.
Films of even 50 micron thickness, however, tend to separate from the support under high temperature cycling stress, and would still provide a high electrical resistivity. What is needed is a simplified method of forming continuous sodium Beta-alumina films at temperatures below 1500.degree. C. The films should have thicknesses of between about 0.005 micron to about 30 microns, and require no high temperature sintering. The pore size of the film should be such that no non-ionic sodium permeation can occur, even though the film is below 30 microns thick. The method should provide films of low porosity having decreased total electrical resistivity.